The present invention relates to improvements in batteries and, more particularly, to a carbonaceous additive for batteries to be combined with the active battery material to improve the service performance of the battery.
Carbon additives are used in many types of batteries (e.g., alkaline, lithium primary, lithium ion, lithium polymer, and rechargeable lead-acid batteries) to enhance the conductivity of the active material. For example, in a typical alkaline battery, the cathode includes manganese dioxide (MnO2) particles as an active material in combination with graphite, potassium hydroxide (KOH) solution, and deionized water. The MnO2 formed by electrolysis (commonly called electrolytic manganese dioxide or EMD) has an extremely low of specific conductivity, so graphite is added to enhance the electrical conductivity, both between the MnO2 particles and between the cathode and the steel can forming the battery. (See, e.g., U.S. Pat. No. 5,489,493 to Urry, which discloses the use of synthetic or natural graphite as the xe2x80x9cconductorxe2x80x9d in the cathode.)
Various graphite additives have been proposed for enhancing this service performance of batteries. For example, U.S. Pat. No. 5,482,798 to Mototani, et al. disclose the use of expanded graphite particles having an average particle size in the range from 0.5 to 15 mm, the expanded graphite comprising between 2 to 8 wt % of the solids in the mixed cathode active material. Synthetic or artificial graphite is preferred by Mototani, et al. because of its lower level of impurities, such as iron. See also, International Publication WO 99/34673, which discloses an electrochemical cell in which the cathode includes expanded graphite particles having an average particle size between 17 to 32 mm and having kerosene absorption value in the range of 2.2 to 3.5 ml/g. The surface area to mass ratio, tap density, Scott density, and the purity level of the expanded graphite are also specified. An expanded graphite having these characteristics, and the method of producing such an expanded graphite art, are disclosed in the co-pending applications Ser. No. 09/253,957, filed Feb. 22, 1999, now U.S. Pat. No. 6,287,694, incorporated by reference herein, and having the same assignee as the present application.
While the use of such expanded graphite additives resulted in impressive improvements in the service performance of electrochemical cells, further improvements in battery service performance within the bounds of economic feasibility are always desirable.
Thus, it is the principal object of the present invention to provide for the improved service performance of electrochemical cells.
More particularly, it is the object of the present invention to provide a carbonaceous material to be mixed with the active material in the formation of the cathode for a battery.
These objects, as well as other that will become apparent upon reference to the following detailed description and accompanying drawings, are achieved by an engineered carbonaceous material (ECM) comprising a mixture of synthetic graphite and one or more other graphite, such as natural flake graphite, natural vein graphite, and/or amorphous graphite. The objects are also achieved by an ECM comprising a mixture of expanded graphite and one or more other graphite materials, such as natural flake graphite, natural vein graphite, amorphous graphite and/or synthetic graphite, in which the carbonaceous material has a purity of between 90.0 and 99.9% C (based on LOI). The ECM is preferably mixed with 0.01 to 20.0 wt % MnO2 to create a battery active material. The mixtures may be made by either co-blending or co-grinding the graphites together. The mixtures may contain between 0.1 and 99.9 wt % synthetic graphite and may be further combined with from between and including 92.0-95.2 MnO2. An electrochemical cell incorporating the material is also contemplated.